Conventional slot nozzle coating of molten thermoplastic compositions onto substrates is typically done by keeping the slot nozzle in contact with the substrate such that the nozzle lies on the substrate during the coating. It is unproblematic to coat hot melt adhesives at low coating weights provided that the coating need not be completely closed, i.e. nonporous. In the context of this specification, “continuous” may be used to describe a completely closed, i.e. nonporous film or coating. If, however, a completely closed, i.e. nonporous coating is to be created, this can only be done using customary coating methods if the coating weight of the hot melt is substantially higher.
Such high coating weights are expensive. Furthermore, direct coating with a slot nozzle provides substantial mechanical and thermal stresses on the coated substrates, especially since the slot nozzle is heated during coating. Therefore, very sensitive substrates such as plastic films can not always be coated with a hot melt from a slot nozzle in a customary manner without damaging the substrate. Further, the high coating weights of this prior art lead to increased stiffness of the coated substrate.
WO 96/25902, published Aug. 29, 1996, assigned to the H.B. Fuller Co. in St. Paul, Minn. teaches a method of coating wherein certain thermoplastic compositions, are thermally made flowable and released from a coating device as a continuous coating without contact between the coating device and substrate being coated.
The present invention resides in specific adaptations to this novel coating method for use in a variety of other applications involving coatings on nonporous materials and coatings on porous materials. One type of such application is coatings on nonporous materials such as films. Thermoplastic compositions often contain unmelted particles either in the form of impurities such as contaminants and char or alternatively in the form of a particulate ingredient such as filler and additives. When these particles are of appreciable size and/or the slot nozzle has a relatively small gap, the particles tend to accumulate in the coating device interfering with the deposition of the coating. The particles block the passage of thermoplastic material causing a corresponding striation or streak to form on the substrate being coated. This problem is particularly prevalent in the formation of very thin coatings particularly when the optical quality is of importance such as for high quality graphic art applications, especially where films have to be coated. Accordingly, industry would find advantage in a coating method which rectifies these problems.
It is therefore an important object of this invention to provide a new coating method especially suited for coatings on films, foils, paper and other such materials, which makes it possible to avoid streaking and striation problems, especially at very low coating weights.
It is another important object of the ivention to provide a coating method which permits laminations and coatings to be carried out “inline” or “offline”, using thin films, metallized foils, heat-sensitive materials and other sensitive substrates at reduced risk of obtaining faulty or flawed products.
It is yet another important object of the ivention to make film-to-film and film-to-foil laminations available which do not require the use of reactive adhesives.
Another object of the invention is to provide improved coating methods for coating thermoplastic compositions, especially hot melt adhesives, onto porous substrates such as textiles.
These and other objects and advantages of the invention will become more apparent from the following discussion.